I don't work for FMC, but do use Fresenius machines (H & K) in my acute setting; I'm still new, and much more familiar with the Cobe machines I used to work with in the past, but the principle is the same (the most confounding thing about Fresenius machines are actually the blood line with 3 parts and all those clamps I forget to clamp!
Basically, the NS infusion has nothing to do with a stopped blood pump. NS will infuse if you forget to clamp the saline clamp(s), as may happen - for instance - when you restart the machine after infusing a few cc of NS after the blood pump stops for high AP (and, more likely, at the beginning of tx if you just forgot to close the clamps). If the clamps are open while the blood pump is stopped, NS can infuse through the arterial line via gravity.
This happens (it just did to a coworker with 8 years experience! She gave a few cc of NS to restart the machine, and 700 cc infused). She was able to increase the UFR without having to extend the tx time since there was enough time left and the pt tolerated the higher UFR; otherwise, she would have added time as well.
P.S. I only know this much about the Phoenix machine: our local DaVita clinics are replacing their depreciated Cobes with Fresenius
(yes, the main competitor's) machines; they don't consider the Phoenix to be safe. Imagine that!